marshal.sh

# Requirements for both hosts: bash, ssh, netcat

PIPE_PORT=8842
TMPDIR=$(mktemp -d)
REQ=$TMPDIR/in
REQO=$TMPDIR/out
DEBUG_LOG=true

trap 'rm -rf "$TMPDIR"' EXIT INT TERM HUP

log () {
	if [[ $DEBUG_LOG = true ]]; then echo "$1" >&2; fi
}

# Initiates the marshal connection and bootstraps the execution queue
marshalconnect () {
	# Create stdin/stdout and back-pipe fifos for the remote execution queue
	mkfifo $REQ
	mkfifo $REQO

	log "ssh fifo setup: $REQ"
	
	# The remote execution queue is bash accepting commands over an ssh connection.
	# Redirect stdin to a fifo and fork ssh
	# TODO: use stdout for communicating exit codes?
	ssh -qC -R $PIPE_PORT:localhost:$PIPE_PORT < $REQ > $REQO $1 /bin/bash &
	SSH_PID=$!

	# Open stdin and stdout fifos for REQ
	log "Opening REQ fifos"
	exec 3>$REQ
	exec 4<$REQO

	# Bootstrap the execution queue
	log "bootstrapping"
	echo "PIPE_PORT=$PIPE_PORT; $(declare -f log bootstrap); bootstrap" > $REQ

	log "REQ started"
}

# Bootstrap function that runs on the remote execution queue to
# set up the execution environment
bootstrap () {
	local tmpdir=$(mktemp -d)
	BPIPE=$tmpdir/back
	trap 'rm -rf "$tmpdir"' EXIT INT TERM HUP

	# Create a fifo for use by the two-way netcat pipe
	mkfifo $BPIPE
}

# Tell the remote session to terminate and then kill our ssh tunnel
marshaldisconnect () {
	echo 'exit 0' > $REQ
}

marshal () {
	# Wrap the command and args sent to this function ($@) to pipe our
	# stdin/stdout over a two-way netcat
	local cmd="$@ <\$BPIPE | nc localhost $PIPE_PORT >\$BPIPE"
	#local cmd; printf -v cmd "%q " "$@"; cmd+=" <\$BPIPE | nc localhost $PIPE_PORT >\$BPIPE"

	local funcname=$1

	# If funcname is a function then define the function on the remote end before executing it
	if [ "$(type -t $funcname)" = function ]; then
		cmd="$(declare -f $funcname); $cmd"
	fi

	# Send the command to the execution queue
	log "Command Start: $cmd"
	echo "$cmd" > $REQ

	# The remote command connects to this waiting netcat to pipe our stdin/stdout
	# over the ssh forwarded port. This will block until the remote command terminates
	# and closes the netcat connection.
	#
	# Stderr is kindly transported by the ssh process for us
	log "Command Run"
	nc -l $PIPE_PORT
	
	log "Command Complete"
}

test.sh

#!/bin/bash
. marshal.sh

marshalconnect $1

marshal BLAH='blah'
marshal 'echo $BLAH'

marshaldisconnect